The present invention relates generally to plasma processing and, more particularly, to plasma processing chambers and electrode assemblies used therein. Plasma processing apparatuses can be used to process substrates by a variety of techniques including, but not limited to, etching, physical vapor deposition, chemical vapor deposition, ion implantation, resist removal, etc. For example, and not by way of limitation, one type of plasma processing chamber contains an upper electrode, commonly referred to as a showerhead electrode, and a bottom electrode. An electric field is established between the electrodes to excite a process gas into the plasma state to process substrates in the reaction chamber. Embodiments of plasma processing chambers and showerhead electrode assemblies used therein are described in U.S. patent application Ser. No. 11/871,586, filed Oct. 12, 2007, the entire disclosure of which is hereby incorporated by reference.